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I’m considering buying a Midland GXT1000VP4 radio set from amazon here: https://www.amazon.com/dp/B001WMFYH4/ref=cm_sw_r_cp_api_6BfmBbBQZ5VBY

In Suburban Pennsylvania how much range could I expect from these walkie-talkies? Could I possibly open one/both up and attach an external antenna to extend the range?

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In Suburban Pennsylvania how much range could I expect from these wallow talkies?

Well, this if very hard to answer, other than "it depends". If you have an absolute clear day, high pressure, no obstruction (what so ever, not even a bird flying), you may get 12 miles or even more, hilltop to hilltop. However this would be rare and an exception to the normal performance. As terrain and environment changes, you would expect 1-2 mile radius, and even that cannot be guaranteed.

It would be a hit and miss at a distance of 12 miles.

Could I possibly open one/both up and attach an external antenna to extend the range?

You could, but you will be breaking the law. Under FCC rules for FRS the unit need to be type approved, and with that comes the stock antenna. The moment you open it up and tamper with the antenna (read: connect your own antenna) you will break this type-approval.

Although the following picture is technically breaking the rules (as you increase EIRP) you will get a more directional performance out of your handset without opening and tampering:

enter image description here (source: http://on3jt.byze.be/build-a-lowcost-yagi-antenna-for-pmr446)

If you have a GMRS license, you will be able to use higher power outputs, and your chances will be improved. See here for a table with allowed power outputs.

Unfortunately the radio you mention does not lists its power outputs in the manual, I suggest that you contact technical support to get more information

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    $\begingroup$ +1 for ingenuity :) $\endgroup$ – Phil Frost - W8II Jun 25 '18 at 13:10
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These radios will operate largely on a line of sight basis with a slight boost from refraction. This is known as the radio line of sight. It is given as:

$$D\approx1.41\sqrt{H} \tag 1$$

where D is the distance in miles and H is the antenna height in feet. Any difference in elevation should be included in H. Note that D is only an approximation since the earth is not perfectly round and the atmospheric conditions affect the refraction.

So if you are holding the radio at a height of 5 feet and there are no obstructions (trees, buildings, hills, etc.) between you and the other radio that is at ground level, the range would be ~3.2 miles assuming both radios locations are at the same earth elevation. If the other radio is also held at 5 feet, the range would nearly double.

To get to your desired 12 mile range, each radio would need to be at a height of a little over 18 feet, for example - provided that there are no obstructions between the radios at this height.

Changing the antenna would not substantially increase the range since the limiting factor is the line of sight. You should also note that FCC regulations prohibit changing the antenna.

Formula 1 also points out the marketing centric nature of the 36 mile distance claim. While this figure is likely based on a link budget, it is not easily attained unless you have the fortuitous situation of having the equivalent of each radio at ~165 feet with no obstructions in-between.

In Amateur Radio and GMRS services, repeaters are used to boost range. The repeater antenna is usually at a significant elevation giving the repeater a large radio horizon. The repeater picks up the signal and retransmits it on another frequency thereby greatly increasing the effective range of individual radios. You can read more about GMRS repeaters at https://www.mygmrs.com.

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